The present invention relates to engine start systems and, more particularly, to engine start systems that may use a readily available constant frequency power source such as an auxiliary power unit (APU) or external ground power.
Using electric, brushless starter-generators for main engine start and for power generation can save aircraft weight and improve operating economics as part of the new more electric architecture (MEA) systems architectures. The starter-generator has to provide high starting torque to start the main engine. Conventional main engine starter-generator systems involve the use of high power inverters to provide a controllable variable frequency power source which adds cost, weight and complexity while decreasing the reliability of the system.
Many aircraft include starter-generator systems to supply relatively constant frequency AC power. Many of the starter-generator systems installed in aircraft include three separate brushless starter-generators, namely, a permanent magnet starter-generator (PMG), an exciter, and a main starter-generator. The PMG includes a rotor having permanent magnets mounted thereon, and a stator having a plurality of windings. When the PMG rotor rotates, the permanent magnets induce AC currents in PMG stator windings. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current to the exciter.
The exciter typically includes single-phase (e.g., DC) stator windings and multi-phase (e.g., three-phase) rotor windings. The DC current from the regulator or control device is supplied to exciter stator windings, and as the exciter rotor rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the main starter-generator. The main starter-generator additionally includes a rotor and a stator having single-phase (e.g., DC) and multi-phase (e.g., three-phase) windings, respectively. The DC currents from the rectifier circuits are supplied to the rotor windings. Thus, as the main starter-generator rotor rotates, three phases of AC current are induced in main starter-generator stator windings. This three-phase AC current can then be provided to a load such as, for example, electrical aircraft systems.
Many of these starter-generator systems are driven by variable speed prime movers. For example, many starter-generators are driven by the aircraft engines, which may vary in rotational speed during operation. Thus, to ensure the AC starter-generators supply relatively constant frequency AC power, many aircraft include a hydro-mechanical transmission, or other type of gear arrangement, that converts the variable engine speed to a relatively constant rotational speed.
Although the above-described configuration is generally safe, hydro-mechanical transmissions can be relatively large, heavy, complex, and/or may exhibit relatively poor reliability. Each of these factors can lead to high overall aircraft, fuel, and maintenance costs, and/or increased maintenance frequency, which can further lead to increased costs.
U.S. Pat. No. 6,188,204 to Vithayathil et al. disclose employing main windings and auxiliary windings disposed on the same rotor. The auxiliary windings are supplied with adjustable frequency AC power, and in turn excite the main windings to produce a desired three phase output frequency. Although this solution does work, the main and auxiliary windings must be arranged to be magnetically decoupled by configuring a specified number of poles, so that the flux generated by the main windings does not induce any voltage in the auxiliary windings that has a different number of poles or vice versa. This can lead to complexity in design and implementation.
U.S. Pat. No. 7,064,455 to Lando employs rotor windings of the exciter and the main starter-generator disposed on the same primary shaft, with a permanent magnet starter-generator (PMG) disposed about an associated secondary shaft, for determining the rotational speed of the primary shaft. The rotors of the exciter and main starter-generator employ three-phase windings. However, this design offers no improvement over the complexity inherent in such three-phase windings.
Both patents mentioned above concern generation mode only and have no provision to be a starter. Hence, it can be seen that there is a need for a system and method of starting a main engine from a starter-generator that is relatively small, lightweight, less complex, and more reliable, as compared to current systems and methods, and that does not rely on specified numbers of exciter and main starter-generator poles.